move-only: test: make snapshot chainstate setup reusable

Summary:
For use in next commit.

Most easily reviewed with
--color-moved=dimmed_zebra --color-moved-ws=allow-indentation-change.

This is a partial backport of core#25667
https://github.com/bitcoin/bitcoin/pull/25667/commits/3a29dfbfb2c16a50d854f6f81428a68aa9180509

Depends on D14651

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14652